The Chamber of Secrets is a legendary secret room constructed by Salazar Slytherin to house a monster which would finish his "noble work." This noble work would be accomplished when his true heir would open the Chamber and release the monster which lived within to rid the school of non-pureblood students.
According to the legend, Slytherin and Gryffindor argued over whether to allow Muggle-born students into Hogwarts. When Gryffindor apparently won, Slytherin left the school, but not before building the Chamber. Over the centuries, many great witches and wizards have searched for the Chamber but none found it. Eventually, the whole thing was assumed to be purely fiction.
The Chamber of Secrets is well hidden. The entrance is behind a sink in a bathroom on the second floor. There is a tiny snake figure scratched into the tap and when someone orders "open up" in Parseltongue, the sink recedes and a pipe appears. Sliding down this pipe takes one to an underground passage, probably located under the lake. There are bones of small animals here.
The Chamber itself is a huge temple-like room with pillars and carved serpents. There is a massive statue of Salazar Slytherin at the far end
